over on the official blog. In any Steam game with the Steam overlay running, you can hit a hotkey (default F12) to snap a screenshot, and then save and publish them off to your Steam Community profile, or save them on social networks like Facebook and Twitter.
Once taken, you can access "Screenshots" under the "View" menu in Steam, and the app even lets you access them directly on your hard drive via a "Show on Disk" button. It's a very groovy way to integrate something that not all games make clear. Now, how long until we can capture video like this?